riscv: add support for multi-hart systems

On RISC-V, all harts boot independently. To be able to run on a
multi-hart system, U-Boot must be extended with the functionality to
manage all harts in the system. All harts entering U-Boot are registered
in the available_harts mask stored in global data. A hart lottery system
as used in the Linux kernel selects the hart U-Boot runs on. All other
harts are halted. U-Boot can delegate functions to them using
smp_call_function().

Every hart has a valid pointer to the global data structure and a 8KiB
stack by default. The stack size is set with CONFIG_STACK_SIZE_SHIFT.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
index 86136f5..644e6ba 100644
--- a/arch/riscv/include/asm/csr.h
+++ b/arch/riscv/include/asm/csr.h
@@ -46,6 +46,7 @@
 #endif
 
 /* Interrupt Enable and Interrupt Pending flags */
+#define MIE_MSIE	_AC(0x00000008, UL) /* Software Interrupt Enable */
 #define SIE_SSIE	_AC(0x00000002, UL) /* Software Interrupt Enable */
 #define SIE_STIE	_AC(0x00000020, UL) /* Timer Interrupt Enable */